There is a general need for energy storage technologies, such as to help balance supply and demand in the electric power grid. This need becomes more prominent with the increasing use of renewable energy sources such as wind and solar, because the pattern of energy generation from those sources tends to be irregular in time and not necessarily well matched to demand.
Currently, the energy storage technology that has by far the largest base of experience at significant size scales is pumped storage hydro. Conventional pumped storage hydro uses two reservoirs of water at different elevations. However, new sites for pumped storage hydro are often non-optimal for reasons of topographical location, environmental sensitivity or significant capital expenditure. It would be desirable to allow the use of pumped storage technology in more locations including at sites that are not mountainous.
A known effort of this type is published by Olsen as US patent application 20110113769, and WO2010003412A2, and also “A new principle for underground pumped hydroelectric storage,” by Jan Olsen, Journal of Energy Storage 2 (2015) 54-63). In this system, hydraulic head is created by the weight of soil on top of a deformable bag. The soil has a density greater than the density of water. When the bag is filled with a fluid such as water so as to store energy, the soil on top of the bag is lifted upward. For recovery of energy, the fluid such as water is allowed to flow out of the bag as the soil on top of the bag settles to a lower elevation. Energy storage is accomplished using a pump, and energy recovery is accomplished with a turbine-generator. Similar technology is described in U.S. Pat. No. 8,950,181 to Ivy et al. In general, it is desirable to provide further improvements in energy storage by pressurized water stored in bags, including improvements relating to bag design and placement.